Methods and apparatus for optically measuring polarization rotation of optical wavefronts using rare earth iron garnets

• US 6,534,977 B1
• Filed: 10/21/1999
• Issued: 03/18/2003
• Est. Priority Date: 10/21/1998
• Status: Expired due to Fees

• Current Assignee: Carvel E. Holton, Paul Duncan, Richard O. Claus
• Original Assignee: Carvel E. Holton, Paul Duncan, Richard O. Claus
• Inventors: Duncan, Paul, Claus, Richard O., Holton, Carvel E.
• Primary Examiner(s): Lefkowitz, Edward
• Assistant Examiner(s): Zaveri, Subhash
• Application Number:
  US09/421,399
• Time in Patent Office: 1,244 Days
• Field of Search: 324/244.1, 324/96, 324/750, 324/752, 250/227, 250/225, 359/280, 359/281, 359/282, 359/283
• US Class Current: 324/244.1
• CPC Class Codes: G01R 15/245 using magneto-optical modulators, e.g. based on the Faraday or Cotton-Mouton effect
• CPC Class Codes: G01R 15/246 based on the Faraday, i.e. linear magneto-optic, effect
• CPC Class Codes: G01R 33/0322 using the Faraday or Voigt effect
• CPC Class Codes: G02B 6/42 Coupling light guides with opto-electronic elements
• CPC Class Codes: G02B 6/4207 with optical elements reducing the sensitivity to optical feedback anti-reflection devices specially adapted for lasers, see H01S3/0064
• CPC Class Codes: G02B 6/4214 the intermediate optical element having redirecting reflective means, e.g. mirrors, prisms for deflecting the radiation from horizontal to down- or upward direction toward a device G02B6/4246 takes precedence
• CPC Class Codes: G02B 6/4216 incorporating polarisation-maintaining fibres polarisation-maintaining fibres per se G02B6/105

Abstract

Described are the design of a rare earth iron garnet sensor element, optical methods of interrogating the sensor element, methods of coupling the optical sensor element to a waveguide, and an optical and electrical processing system for monitoring the polarization rotation of a linearly polarized wavefront undergoing external modulation due to magnetic field or electrical current fluctuation. The sensor element uses the Faraday effect, an intrinsic property of certain rare-earth iron garnet materials, to rotate the polarization state of light in the presence of a magnetic field. The sensor element may be coated with a thin-film mirror to effectively double the optical path length, providing twice the sensitivity for a given field strength or temperature change. A semiconductor sensor system using a rare earth iron garnet sensor element is described.

Claims

1. A magneto-optic sensor element comprising:

a crystal substrate;

a thin-film reflective surface on a first face of said crystal substrate;

a rare-earth iron garnet thin-film on a second face of said crystal substrate, said second face opposing said first face; and

,

an anti-reflection coating on said rare-earth iron garnet substrate the anti-reflection coating has a thickness t which is within the range;

0<

=t<

N* .lambda. /4 where lamda represents the primary wavelength of an incident polarized wavefront and N is an odd- integer such that 1<

=N<

=∞

.

2. The magneto-optic sensor element according to claim 1, wherein said thin film reflective surface comprises a dielectric mirror deposited on said crystal substrate.

3. The magneto-optic sensor element according to claim 1, wherein said thin film reflective surface comprises a metallic mirror deposited on said crystal substrate.

4. The magneto-optic sensor element according to claim 1, wherein said crystal substrate is optically transparent with respect to the wavelength of an incident polarized wavefront.

5. The magneto-optic sensor element according to claim 1, wherein said rare-earth iron garnet thin-film is deposited on said second face of said crystal substrate.

6. The magneto-optic sensor element according to claim 1, wherein said rare-earth iron garnet thin-film is grown on said second face of said crystal substrate.

7. The magneto-optic sensor element according to claim 1, wherein said anti-reflection coating is deposited on said rare earth iron garnet substrate.

8. The magneto-optic sensor element of claim 1, wherein said thin-film reflective surface doubles the total single-direction optical path length through said rare-earth iron garnet thin film and supporting crystal substrate.

11. The magneto-optic sensor element according to claim 1, further comprising:

a graded-index lens for coupling optical energy into said crystal substrate.

12. The magneto-optic sensor element according to claim 11, wherein said graded-index lens is optically tuned to the quarter wavelength of an incident polarized wavefront from a sensor light source.

13. The magneto-optic sensor of claim 12, wherein said graded-index lens comprises a quarter-pitch lens at the primary wavelength.

14. The magneto-optic sensor of claim 11, wherein said graded-index lens is in the shape of a right-angled cylinder and is bonded to an anti-reflection side of said crystal substrate via an optically transparent epoxy.

15. The magneto-optic sensor element of claim 11, wherein said graded-index lens is polished on at least one end to a facet angle of 0<

=alpha. <

=11 degrees, measured with respect to the rotational symmetry axis of the cylinder.

16. The magneto-optic sensor of claim 11, arranged such that an incident plane polarized light beam propagating through said sensor element travels along the axis of said graded-index lens, through said crystal substrate, and strikes said thin-film mirror at essentially normal incidence.

9. A magneto-optic sensor element comprising:

a crystal substrate which is optically transparent with respect to the wavelength of an incident polarized wavefront from a sensor light source, said crystal substrate further comprising a rare-earth iron garnet crystal;

a dielectric thin-film mirror deposited one side of said rare-earth iron garnet crystal; and

,

an anti-reflection coating of thickness 0<

=t<

=N*.lambda./4, where lambda is the primary wavelength of an incident polarized wavefront and N is an odd-integer multiple such that 1<

N<

=∞

, deposited on the opposite end of the rare-earth iron garnet substrate.

10. The magneto-optic sensor element of claim 9, wherein said thin-film reflective surface doubles the total single-direction optical path length through said rare-earth iron garnet thin film and supporting crystal substrate.

17. A magneto-optic sensor probe comprising:

a crystal substrate;

a thin-film reflective surface on a first face of said crystal substrate;

a rare-earth iron garnet thin-film on a second face of said crystal substrate, said second face opposing said first face;

a graded-index lens for coupling optical energy into said crystal substrate; and

,

a optical fiber coupled with said graded-index lens.

18. The magneto-optic sensor probe of claim 17, wherein said optical fiber comprises a polarization-maintaining single mode optical fiber.

19. The magneto-optic sensor probe of claim 18, further comprising an anti-reflection coating on said rare-earth iron garnet substrate.

20. The magneto-optic sensor probe of claim 18 wherein said polarization maintaining single mode optical fiber is bonded to said graded-index lens using optical epoxy.

21. A fiber optic sensor system, comprising:

a light source for emitting a light beam;

a polarizing means for polarizing said light beam;

fiber optic coupler;

crystal substrate having a rare-earth iron garnet thin-film on a face thereof;

graded index lens assembly optically coupled to said fiber optic coupler;

beamsplitter optically coupled to said graded index lens assembly; and

,

detector means for converting optical energy into electrical energy.

22. The fiber optic sensor system according to claim 21, wherein said polarizing means comprises a Faraday isolator.

23. The fiber optic sensor system of claim 22 wherein:

said light source is coupled to said optical fiber via a lensing system such that the output of said light source is effectively coupled into a core of said optical fiber;

said light source outputs an arbitrary state of polarization;

said light source is fusion spliced to an input of said Faraday isolator;

said Faraday isolator polarizes and rotates said arbitrary state of polarization at said isolator input to a known state of polarization;

said fiber optic coupler comprises a 2×

2 polarization-maintaining single mode coupler which is arranged such that the power ratio between output arms of said coupler is 1;

1 and such that the known state of polarization produced by said Faraday isolator is maintained in each output arm;

one output arm of said fiber optic coupler is terminated into a forward power monitoring photodiode;

another output arm of said fiber optic coupler is fusion spliced to said optical fiber, said optical fiber being optically coupled to said crystal substrate;

the remaining arm of said fiber optic coupler is coupled to said graded index lens;

the output of the said beamsplitter having two independent optical intensities;

said signal recovery photodiodes being positioned such that the output from the said beamsplitter is incident upon their active regions.

24. The fiber optic sensor system according to claim 21, wherein said fiber optic coupler comprises a 2×

2 fiber optic coupler.

25. The fiber optic sensor system according to claim 21, wherein said detector means comprises two signal recovery photodiodes.

26. The fiber optic sensor system according to claim 21, wherein said light source comprises a laser light source.

27. The fiber optic sensor system according to claim 21, wherein said light source comprises an LED light source.

28. The fiber optic sensor system according to claim 21, further comprising:

a forward power monitoring photodiode into which an output arm of said fiber optic coupler is terminated.

29. The sensor system of claim 21 wherein said beamsplitter is a polarization beam splitter (PBS) providing at least a 500:

1 extinction ratio between its two output intensities.

30. The sensor system of claim 28, wherein said forward monitoring power diode provides additional compensation and noise reduction in the drive electronics of said light source.